Prospect of endophyte mediated disease management in mulberry

J. Justin Kumar

Mr. J. Justin Kumar works with Central Sericultural Research and Training Institute, Mysore, India. His research interests include silkworm pathology and application of bio-molecules in silkworm disease management. Mr. Justin Kumar can be contacted at  justinkumarj@gmail.com

Introduction

Endophytes are fungi or bacteria occurring inside plant tissues without showing any apparent symptoms by the host (Wilson, 1995). Together with mycorrhizal fungi, endophytes form an integral part of the extended phenotype or symbiotic community of a plant (Whitham, et al., 2003). They are ubiquitous and the association of endophytic organisms with their host plants is intricate (Tadych and White, 2009). A single leaf or root of a plant can harbor different species of endophytes. Thousands of endophytes are reported to be useful to mankind (Tan and Zou, 2001; Strobel, 2002; Strobel and Daisy, 2003). Many compounds of endophytic microbes have been used in agriculture, medicine, and industry.  Endophytes also play important roles in the ecology of plant communities, resistance to insects and diseases, increase plants’ capacity to absorb water and minerals, and augment their ability to withstand dry periods.

Sericulture could immensely benefit from the endophytes. Providing quality feed, standard rearing conditions, protection from diseases and pests to the host plant as well as to the insect are the basic conditions for successful silkworm rearing. In this regard, the endophytes can play a vital role in the quality leaf production from mulberry (Morus alba L.), the sole food plant of the silkworm (Bombyx mori L). Probably the first report of endophyte isolated from mulberry is by Sato, et al., (2000), and the bacterial strain they reported Xanthomonas compestris was non pathogenic one. Improved plant growth, higher nutrient content, resistance to insect pests and herbivores, resistance or tolerance to diseases, increased competitiveness, enhanced tolerance to stressful factors such as heavy metals, low pH, high salinity, etc., are the benefits from the endophytic interaction.

Mulberry diseases and its management

Foliar and soil borne diseases are a major constraint in the production of quality mulberry leaf. Major  diseases in mulberry are the following

Cultural, mechanical, agrochemical and biological approaches employed to control the mulberry diseases individually or in combination as integrated approach. Though agrochemicals are effective for the disease control, the residual toxicity to the silkworm and environmental constraints have made it imperative to search for biological remedial measures. In this direction a lot of antagonistic microbes were studied for the control of diseases. But these microbes are affected by biotic and abiotic factors and its effectiveness has been inconsistent. Under these circumstances, the endophytes have received much attention for its use as biocontrol agents. The advantage is that, once inside the host, they are better protected against environmental stress and microbial competition. Further, some of the endophytes produce endospores, which can be formulated and stored (Ji, et al., 2008). Studies have been conducted on the application of endophytic Bacillus strains for the control of soilborne pathogens in many agricultural crops.

It is interesting to note that in many other crops/plants, the endophytes are being isolated and tested and found effective against pathogenic microbes, which is indicated in the table.

Prospects in mulberry disease management

Ji, et al., (2008) have made an attempt to control Ralstonia solanacearum (Smith) the soil borne bacteria, causing wilt disease in mulberry. They showed that the Bacillus subtilis strain Lu144 effectively reduced the bacterial wilt of mulberry when it was applied to sterile or non sterile soil before the infection by the pathogens. Mu, et al., (2008) have succeeded in colonizing an antagonistic bacteria Burkholderia cepacia strain Lu10-1, isolated from healthy mulberry by acupuncturing, seed soaking, root soaking and leaf daubing. They suggested that this strain can play an important role in the biological control of mulberry diseases.  Ji, et al., (2010) have reported that the B. cepacia strain Lu10-1 can multiply and spread in mulberry seedlings rapidly and efficiently. They found that the strain is antagonistic to Colletotrichium dematium, the anthracnose causing pathogen. It is an indication that there is a lot of scope for further studies for effective utilization of endophytes of mulberry or other plants in the mulberry disease management. The possible challenges may be

• Identification and isolation of suitable endophytes
• Colonization of such endophytes to mulberry
• Safeguard against loss of palatability to silkworm during such colonization 

Though the endophytic studies have gained momentum in the recent past, it holds a lot of promises for resolving many problems confronted in the agriculture and medical fields. Though a large number of endophytes are host specific and unique, there are a number of endophytes which can be found in many species of plants, and hence holds a promise that such candidates are accessible for experimenting in the sericulture field, whether for host plant improvement of silk insects or on the silk insects itself! Search for novel endophytes existing in the host plants, enumerating and striking its potential holds the key to success.

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